Operational amplifiers (often abbreviated “op amps”) are common semiconductor parts often configured to provide amplification of a comparative or differential signal represented by the difference between inputs on a positive (“+”) pin and a negative (“−”) pin. Op amps are often used to amplify low-level signals. In order to achieve better amplification results, particularly at the limits of amplifier sensitivity, it is useful for the manufacturer of op amp parts to be able to identify the inherent leakage current, or input bias current, that flows into the op amp inputs in the absence of an input signal.
It is known to attempt to measure the input bias current (sometimes referred to as MB current) of op amps or other parts using various techniques. As shown in FIG. 1A, in some conventional testing platforms, referred to as pico-amp meters (PAMs), the op amp or other device under test is connected to a much higher sensitivity op amp, such as an instrumentation op amp, by a lead wire leading from the op amp under test to the remotely attached instrumentation amp. The remote amp can serve to convert the input bias current to voltage, and measure the value of the input bias current over time based on the converted voltage. In conventional configurations of this type, although the remote instrumentation amp may have higher sensitivity than the op amp under test, the attachment of lead wires, which can be up to several inches in length or more, can produce stray capacitance, leakages, or other electrical effects which offset or distort the high-sensitivity measurement by the remote PAM. These unintended effects can skew or invalidate the input bias current results. While some of these effects can be measured and calibrated out, those leakages and other effects may not be stationary over time, creating a measurement floor due to that drift.
As shown in FIG. 1B, in other known op amp test platforms, a servo loop can be added in parallel to the actual device under test which integrates the input bias current over time via a sensing capacitor. In this arrangement, known as a remote integrator, the servo loop create a feedback effect in which the slope of the op amp's voltage output is proportionate to the input bias current, which can then be derived from that captured voltage data. Sensitivity can be adjusted by selecting values for the sensing capacitor and the length of the test run time, and can achieve desirable levels in the femto Ampere (fA) range, sufficient to measure most manufactured op amps at present. However, the use of a remote integrator test arrangement also requires that the device under test be placed in the circuit to actually close the servo loop. The resulting voltage ramp is not, however, guaranteed to produce consistent results depending on the characteristics of each op amp or other device under test. Test platforms using remote integration therefore have required the use of reference or “golden” sample units whose input bias values or other parameters are known, through independent means, to be within desired ranges. This requires the periodic insertion and verification of the remote integrator platform using such reference or correlation parts, which may not be efficient or desirable in a manufacturing or other production environment.
It may be desirable to provide methods and systems for high-sensitivity detection of input bias current which can provide in-circuit input bias current testing capability for op amps or other parts to within a high degree of sensitivity, without the use of long attached connection leads or a requirement for reference parts.